There has been increasing interest in recent years in fuel cells, i.e. electrochemical cells which produce electricity directly from the oxidation of a fuel, such as methanol. These cells require catalysts and their efficiency is directly related to the activity of the catalysts employed. However, minor differences in a catalyst formulation can have a major effect upon the activity of that material as a catalyst, and, since there are many variables even in a simple catalyst, testing a multitude of individual materials for their activity as catalysts is time-consuming and expensive. In addition, transmission of information from a large array of cells requires many wire connections to the cells and many wires to transmit signals between the cells and the monitoring equipment.
A paper entitled “Detection of Catalytic Activity in Combinatorial Libraries of Heterogeneous Catalysts by IR Thermography” [Angew. Chem., Int. Ed. (1998), 37, 2644-7], A. Holzwarth et al., describes the measurement of catalyst activity by measuring temperature change across an array of catalysts exposed to reducing gases mixed with oxygen. Although this technique does not require wire connections and can give a general indication of the chemical activity in oxygen, the results do not apply directly to electrochemical cells where oxygen gas does not make direct contact with the reactant.
In another paper entitled “Automated Electrochemical Combinatorial Electrode Arrays” [Anal. Chem. (1999), 71, 4369-4375], M. G. Sullivan et al. propose the use of a technique based upon the principles of combinatorial chemistry to solve this problem. The proposed technique involves the use of a test electrochemical cell in which one electrode is based upon a structured array of the materials to be tested for their activity as electrocatalysts and the electrolyte is a special electrolyte composition which, due to the electrocatalysis, fluoresces to an extent proportional to the current passed. Whilst this does, indeed, allow for many materials to be tested rapidly and economically, the method interferes with the chemical environment of the electrocatalyst by adding and/or substituting components to the electrolyte solution which are not normally present in the fuel cell. Therefore, the results obtained using such a test cell may not be truly representative of what would be obtained if the same electrocatalysts were used in a fuel cell containing only the desired fuel dissolved in the specified electrolyte.
Finally, a paper entitled “High Throughput Screening System for Catalytic Hydrogen-Producing Materials” [J. Comb. Chem. (2002), 4, 17-22], T. F. Jaramillo et al., describes the use of a Pd-coated tungsten oxide film in a colourimetric method to detect electrolytically generated hydrogen gas from an array of electrocatalysts. In this case the catalytic reaction examined is quite different from the reaction required of a fuel cell electrocatalyst.